1. Field
The invention is in the field of methods and apparatus for determining the remaining charge or useful life (state-of-charge) of an electrochemical battery.
2. State of the Art
Primary electric batteries of different kinds are used for generation of electricity through conversion of chemical energy, from the reaction of their electrodes with the electrolyte solution, into electric current with a certain voltage. Once the reactive mass of the electrodes or the electrolyte is depleted, these batteries are finished and must be replaced. Secondary batteries that can be recharged after depletion are capable of delivering considerable amounts of electricity with high currents and are used in a wide variety of applications. In these batteries, the chemical energy from the reactions between the electrodes and the electrolyte that is spent on producing electricity, can be replenished by reversing the chemical reactions on the electrodes during a recharging process that involves passing an electric current, from another source, through the battery in the reverse direction.
A common feature of most electric batteries is their relatively constant voltage during use. The voltage is not usually dependent on the battery's charge content so an instantaneous measurement of a battery voltage does not relate to the quantity of charge remaining in the battery. Only when the battery gets close to being completely discharged does its voltage start to drop off noticeably. In other words, the battery voltage is not a reliable indicator of its energy content. This is particularly true in some lithium batteries. It is desirable in many cases, and in some situations it is essential, to know exactly how much energy is available in a battery system at any instant during its use. For these reasons, the subject of indicating the remaining charge in electric batteries has received the attention of many inventors and there are a large number of patents dealing with "state-of-charge" indicators for batteries. Examples are U.S. Pat. Nos. 3,484,681, 3,617,850, 3,895,284, 3,898,548, 3,906,329, 4,307,330, 5,315,228, and 5,315,253.
Most of the devices described in the prior art patents measure the electrical signals of a battery cell or measure charging current into a battery and current taken from the battery and, using them in a variety of procedures, estimate how much electric charge is left in the battery at any moment. Examples include measurements of voltage and time-integrated currents that pass through the battery, in charging and discharging situations, along with temperature measurements for applying necessary correction factors. A common disadvantage of most of these patented methods and devices is the necessity of continuously monitoring the current flow into or out of the battery and keeping track (electronic book-keeping) of the past current history of the battery. Further, none of them is a truly instantaneous charge indicator. Another disadvantage of these methods is that they will never be cost effective for usage on very small batteries, known as dry cells, that are used extensively in many instruments and household equipment.
There remains a need for an accurate and inexpensive solution for measuring the charge in a battery that can be applied to different types of batteries and provide an instantaneous read-out of the state of charge in a battery.